Light emitting diode

ABSTRACT

A light emitting diode (LED) of a double hetero-junction type has a light-emitting layer of a GaAlInP material, a p-type cladding layer and an n-type cladding layer sandwiching the light-emitting layer therebetween, a p-side electrode formed on the p-type cladding layer side, and an n-side electrode formed on the n-type cladding layer side. The p-type cladding layer consists of a first p-type cladding layer positioned closer to the light-emitting layer and having a lower aluminum content and a lower impurity concentration, and a second p-type cladding layer positioned less closer to the light-emitting layer and having a higher aluminum content and a higher impurity concentration. The LED also has a current blocking layer below the p-side electrode for locally blocking electric current flowing from the p-side electrode to the n-side electrode.

This application is a division of U.S. Ser. No. 09/761,829, filed Jan.18, 2001, now U.S. Pat. No. 6,621,106.

BACKGROUND OF THE INVENTION

The present invention relates to a light emitting diode (LED) having adouble hetero-structure and more particularly to an LED that has a highoptical power and can be used at a large current.

A highly efficient LED having a double hetero-structure as shown in FIG.21 is known. FIG. 21 is a vertical sectional view showing an AlGaInP LEDin which layers are lattice-matched with a GaAs substrate 1. Thestructure of each layer in the LED is as follows:

-   Substrate 1:    -   made of an n-type GaAs-   Buffer layer 2:    -   made of n-type GaAs-   N-type cladding layer 3    -   made of n-type (Ga_(0.3)Al_(0.7))_(0.5)In_(0.5)P        -   impurity: Si, impurity concentration: 1×10¹⁸ cm⁻³, and        -   thickness: 1 μm-   Light-emitting layer 4:    -   made of p-type (Ga_(0.7)Al_(0.3))_(0.5)In_(0.5)P        -   thickness: 0.5 μm-   P-type cladding layer 5:    -   made of p-type Al_(0.5)In_(0.5)P        -   impurity: Zn, impurity concentration: 5×10¹⁷ cm⁻³, and        -   thickness: 1 μm-   First current diffusion layer 7:    -   made of p-type Ga_(0.3)Al_(0.7)As        -   impurity: Zn, impurity concentration: 1×10¹⁸ cm⁻³, and        -   thickness: 1 μm-   Second current diffusion layer 8:    -   made of p-type Ga_(0.3)Al_(0.7)As        -   impurity: Zn, impurity concentration: 3×10¹⁸ cm⁻³, and        -   thickness: 6 μm-   Contact layer 9:    -   made of p-type GaAs-   An n-side electrode 10 is formed on the underside of the n-type GaAs    substrate 1. A p-side electrode 11 is formed on the p-type GaAs    contact layer 9.

The n-type GaAs buffer layer 2 is intended to eliminate defects of then-type GaAs substrate 1 and influence of contaminants in the substrateand is not required if the n-type GaAs substrate 1 is surface-treatedfavorably. The p-type GaAs contact layer 9 has a Gads structure notcontaining Al to facilitate an ohmic contact between the p-type GaAscontact layer 9 and the p-side electrode 11. The GaAs composing thecontact layer 9 does not transmit light generated from the p-type(Ga_(0.7)Al_(0.3))_(0.5)In_(0.5)P light-emitting layer 4, but no problemis raised because the contact layer is formed immediately below thep-side electrode 11.

Sharp K. K. has recently proposed an LED, a vertical sectional view ofwhich is shown in FIG. 22, to achieve higher reliability than the aboveLED (Japanese Patent Application No. 10-338656). The structure of eachlayer in the LED is as follows:

-   Substrate 21:    -   made of n-type Gas-   Buffer layer 22:    -   made of n-type GaAs-   N-type cladding layer 23:    -   made of n-type (Ga_(0.3)Al_(0.7))_(0.5)In_(0.5)P        -   impurity: Si, impurity concentration: 1×10¹⁸ cm⁻³, and        -   thickness: 1 μm-   Light-emitting layer 24:    -   made of p-type (Ga_(0.7)Al_(0.3))_(0.5)In_(0.5)P        -   thickness: 0.5 μm-   First p-type cladding layer 26:    -   p-type (Ga_(0.5)Al_(0.5))_(0.5)In_(0.5)P        -   impurity: Zn, impurity concentration: 1×10¹⁷ cm⁻³, and            thickness: 0.2 μm-   Second p-type cladding layer 27:    -   made of p-type Al_(0.5)In_(0.5)P        -   impurity: Zn, impurity concentration: 5×10¹⁷ cm⁻³, and            thickness: 1.0 μm-   Current diffusion layer 28:    -   made of p-type Ga_(0.9)In_(0.1)P        -   impurity: Zn, impurity concentration: 1×10¹⁸ cm⁻³, and            thickness: 7 μm-   Contact layer 29:    -   made of p-type GaAs

An n-side electrode 30 is formed on the underside of the n-type GaAssubstrate 21. A p-side electrode 31 is formed on the p-type GaAs contactlayer 29.

A p-type cladding layer 25 is formed as a two-layer structure consistingof the p-type (Ga_(0.5)Al_(0.5))_(0.5)In_(0.5)P first cladding layer 26and the p-type Al_(0.5)In_(0.5)P second cladding layer 27. Accordingly,it is possible to prevent a p-type impurity from diffusing to the p-type(Ga_(0.7)Al_(0.3))_(0.5)In_(0.5)P light-emitting layer 24 although thep-type impurity has a large impurity gradient and is liable to diffusewhen electric current flows through the LED for a long time. Thus it ispossible to prevent deterioration of the optical power of the LED.

The LED is used in the form of a chip. Conventionally, an LED wafer isdivided into chips of a size of 200 μm-300 μm by 200 μm-300 μm. In theabove LEDs, the p-type GaAs contact layers 9, 29 and the p-sideelectrodes 11, 31 are formed circular and disposed at the center of eachchip. FIG. 23 shows a planar configuration of the chip.

The above LEDs have the following problem: Electric current flowsimmediately below the p-side electrodes 11, 31. Both the p-sideelectrodes 11, 31 and the p-type GaAs contact layers 9, 29 disposedunder the p-side electrodes 11, 31 are opaque. Thus, the p-sideelectrodes 11, 31 and the contact layers 9, 29 intercept light comingfrom parts of the p-type (Ga_(0.7)Al_(0.3))_(0.5)In_(0.5)Plight-emitting layers coming from parts of the p-type(Ga_(0.7)Al_(0.3))_(0.5)P light-emitting layers 4, 24 that are locatedimmediately below the p-side electrodes 11, 31. Thus, the light comingfrom those parts cannot be taken out to the outside. Accordingly, theabove LEDs have a low light-emitting efficiency.

The LED chips are conventionally used at electric current LED having anintensity of several milliamperes to 50 mA. If the LED chip is used foran electric current having intensity higher than that, the optical powerof the LED chip will saturate and characteristics will deteriorate withthe passage of a current.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide an LED inwhich light emission immediately below an electrode is restricted toimprove light take-out efficiency so that the LED has an improvedlight-emitting characteristic when it is used at a large current ofseveral milliamperes to 50 mA or more.

In order to accomplish the above object, there is provided, according toan aspect of the invention, a light emitting diode of a doublehetero-junction type comprising:

a light-emitting layer composed of a GaAlInP material;

a p-type cladding layer and an n-type cladding layer sandwiching thelight-emitting layer therebetween;

a p-side electrode formed on the p-type cladding layer side; and

an n-side electrode formed on the n-type cladding layer side;

the p-type cladding layer consisting of a first p-type cladding layerpositioned closer to the light-emitting layer and having a loweraluminum content and a lower impurity concentration, and a second p-typecladding layer positioned farther from the light-emitting layer andhaving a higher aluminum content and a higher impurity concentration;and

a current blocking layer for locally blocking electric current flowingfrom the p-side electrode to the n-side electrode.

The current blocking layer may be provided immediately below the p-sideelectrode which is opaque. With this arrangement, electric currentflowing to those parts of the light-emitting layer that are positionedimmediately below the p-side electrode is restricted. By thussuppressing emission of unrequired light which would be intercepted bythe opaque p-side electrode, it is possible to enhance the lighttake-out efficiency and thus improve the optical power. That is, theexternal light emission efficiency can be enhanced.

If the thickness of the first p-type cladding layer is within a range of0.2 μm to 0.5 μm inclusive, an initial luminous intensity ratio of 100%can be obtained. Thus, it is possible to increase reliability of theLED.

In one embodiment, the p-side electrode has an electrode windowconsisting of an opening, and the current blocking layer has an openingat a position confronting the electrode window of the p-side electrode,and the opening of the current blocking layer serves as a current pathfor intensively passing electric current from the p-side electrodethrough the light emitting diode.

According to the structure, the current density is increased and thusthe internal light-emitting efficiency is also increased. There is afear that the increase of the current density will reduce the opticalpower if electric current is passed through the LED for a long time. Butsuch reduction of the optical power can be suppressed because the p-typecladding layer consists of the first and second layers.

An appropriate current density can be obtained by setting the area ofthe current path to the range of 1,000 μm² to 40,000 μm². Consequently,the internal light-emitting efficiency can be enhanced. In the casewhere the area of the current path is set to the range of 1,000 μm² to20,000 μm², a comparatively dark portion is prevented from taking placein the center of the current path even when the diameter of the currentpath is 150 μm or more. In the case where the area of the current pathis set to the range of 1,000 μm² to 10,000 μm², a high optical power canbe obtained even when the LED is driven at 20 mA.

In one embodiment, the p-side electrode is formed at a central portionof a surface, and the current blocking layer is formed at a positionconfronting the p-side electrode such that electric current coming fromthe p-side electrode flows around of the current blocking layer.

The p-side electrode may be formed at a central part of a surface of alayer. In this case, the LED, which has a high optical power, isfabricated by using the same electrode-forming process as thatconventionally used.

The current blocking layer may be formed inside a current diffusionlayer. In this case, the current blocking layer is located in a positionnearer to the light-emitting layer than when the current blocking layeris formed on the upper surface of the current diffusion layer.Accordingly, it is possible to prevent electric current whose path hasbeen restricted by the current blocking layer from being unfavorablydiffused before it reaches the light-emitting layer.

There is also provided, according to a second aspect of the presentinvention, a light emitting diode of a double hetero-junction type inwhich a light-emitting layer made of a GaAlInP material is interposedbetween a p-type cladding layer and an n-type cladding layer, wherein:

a p-side electrode is formed on a p-type cladding layer-side surfacehaving an area of 0.15 mm² or more; and

any point present in a region not containing the p-side electrode of thep-type cladding layer-side surface is within a distance of (Ld×2) fromsome point on an edge of the p-side electrode, where Ld is a distancefrom a position at which an optical power is maximum, to a position atwhich the optical power attenuates by 90%.

According to the construction, it is possible to obtain a favorablecurrent diffusion and thus, suppress the increase of the currentdensity. Therefore, even if the LED is used at a large electric current,the current density will not become too high. Accordingly, it ispossible to prevent saturation of the optical power of the LED anddeterioration in application of electric current to the LED. Thus, it ispossible to improve the light-emitting characteristic at a largecurrent.

When the distribution of the optical output of an LED chip is examinedalong a line A-A′, as shown in FIG. 17A, passing through a p-sideelectrode 161, the distance Ld is a distance from a position close tothe p-side electrode 161 at which the optical power is maximum, to aposition at which the optical power attenuates by 90% as compared withthe maximum value, as shown in FIG. 17B. Then, according to the presentinvention, as shown in an explanatory illustration of FIG. 18, thep-side electrode (denoted by 162 in FIG. 18) is provided such that anypoint present in a region not containing the p-side electrode of thep-type cladding layer-side surface is within the distance of (Ld×2) fromthe edge of the p-side electrode.

The p-side electrode may comprise a plurality of branch electrodes and aconnection electrode connecting the branch electrodes to each otherelectrically.

In one embodiment, an interval between the branch electrodes isapproximately Ld.

The surface on which the p-side electrode is formed may have two opposedparallel straight sides, and the branch electrodes may be eachstrip-shaped, and arranged parallel with the two sides and with eachother.

In one embodiment, an interval between an outermost branch electrode andthe side of the surface opposed to this branch electrode isapproximately Ld/2.

Furthermore, according to a third aspect of the invention, there isprovided a light emitting diode of a double hetero-junction type inwhich a light-emitting layer made of a GaAlInP material is interposedbetween a p-type cladding layer and an n-type cladding layer,comprising:

a current blocking layer formed on a p-type cladding layer-side surfacehaving an area of 0.15 mm² or more; and

a p-side electrode formed at a position above the current blocking layerand opposed to the current blocking layer,

wherein any point present in a region not containing the currentblocking layer of the p-type cladding layer-side surface is within adistance of (Ld×2) from some point on an edge of the current blockinglayer, where Id is a distance from a position at which an optical poweris maximum, to a position at which the optical power attenuates by 90%.

According to the construction, it is possible to obtain a favorablecurrent diffusion and thus, suppress the increase of the currentdensity. Therefore, even if the LED is used at a large electric current,the current density will not become too high. Accordingly, it ispossible to prevent saturation of the optical power of the LED anddeterioration in application of electric current to the LED. Thus, it ispossible to improve the light-emitting characteristic at a largecurrent.

Regarding the distance Ld, when the distribution of the optical outputof an LED chip is examined along a line B-B′ passing through a p-sideelectrode 163, as shown in FIG. 19A, the distance Ld is a distance froma position close to a current blocking layer 164 at which an opticalpower is maximum, to a position at which the optical power attenuates by90% as compared with the maximum optical power, as shown in FIG. 19B.Then, according to the present invention, as shown in an explanatoryillustration of FIG. 20, the current blocking layer (denoted by 165 inFIG. 20) is provided such that any point present in a region notcontaining the current blocking layer of the p-type cladding layer-sidesurface is within the distance of (Ld×2) from the edge of the currentblocking layer.

In one embodiment, the current blocking layer comprises a plurality ofblocking branch portions and a connection portion connecting theblocking branch portions to each other electrically, and an intervalbetween adjacent blocking branch portions is approximately Ld.

The surface on which the current blocking layer may formed has twoopposed parallel straight sides, and the blocking branch portions may beeach strip-shaped and arranged parallel with the two sides and with eachother.

In one embodiment, an interval between an outermost blocking branchportion and the side of the surface opposed to this outermost blockingbranch portion is approximately Ld/2.

There is also provided, according to a fourth aspect of the presentinvention, a light emitting diode of a double heterojunction type inwhich a light-emitting layer made of a GaAlInP material is interposedbetween a p-type cladding layer and an n-type cladding layer, wherein:

a p-side electrode is formed on a p-type cladding layer-side surface,the p-side electrode consisting of a plurality of mutually connectedconstituent parts; and

any point present in a region not containing the p-side electrode of thep-type cladding layer-side surface is within a distance of (Ld×2) fromsome point on an edge of the p-side electrode, where Ld is a distancefrom a position at which an optical power is maximum, to a position atwhich the optical power attenuates by 90%.

In the LED according to any one of the second through the fourthaspects, if a current diffusion layer made of an AlGaInP material isprovided between the p-type cladding layer and the p-side electrode,electric current favorably diffused by the p-side electrode or thecurrent blocking layer is further diffused positively by the currentdiffusion layer. In this manner, it is possible to obtain more favorablecurrent diffusion.

Also, if a barrier layer having a band gap intermediate between bandgaps of the light-emitting layer and the p-type cladding layer isprovided between the light-emitting layer and the p-type cladding layer,it is possible to prevent a p-type impurity from diffusing to thelight-emitting layer, although the p-type impurity has a large impuritygradient and is liable to diffuse when electric current is passedthrough the LED for a long time. Thus it is possible to preventdeterioration of the optical output of the LED and improve thereliability thereof.

Furthermore, if a barrier layer having a band gap intermediate betweenband gaps of the light-emitting layer and the n-type cladding layer isprovided between the light-emitting layer and the n-type cladding layer,it is possible to prevent an n-type impurity from diffusing to thelight-emitting layer. Thus it is possible to prevent deterioration ofthe optical output power of the LED and improve the reliability thereof.

Other objects, features and advantages of the present invention will beobvious from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

FIG. 1 is a vertical sectional view showing the layer structure of anLED of a first embodiment of the present invention;

FIG. 2 is a plan view showing the LED shown in FIG. 1 in which layersare formed up to a current blocking structure;

FIG. 3 shows dependency of an optical output of the LED shown in FIG. 1on a current density;

FIG. 4 shows the optical output of an LED having a current blockingstructure in comparison with that of an LED having no current blockingstructure;

FIG. 5 is a vertical sectional view showing an LED of a secondembodiment of the present invention;

FIG. 6 shows the relationship between the thickness of a first p-typecladding layer of the LED shown in FIG. 5 and deterioration of the LED;

FIG. 7 is a vertical sectional view showing an LED of a third embodimentof the present invention;

FIG. 8 is a plan view showing the LED shown in FIG. 7;

FIG. 9 is a vertical sectional view showing an LED of a fourthembodiment of the present invention;

FIGS. 10A and 10B are plan views showing the LED shown in FIG. 9 inwhich layers are formed up to a current blocking structure and layersare formed up to a p-side electrode, respectively;

FIGS. 11A and 11B show the relationship between the configurationcharacteristic of a current blocking layer of the LED shown in FIG. 9and the optical output power of the LED;

FIG. 12 is a vertical sectional view showing an LED of a fifthembodiment of the present invention;

FIG. 13 is a plan view of the LED shown in FIG. 12;

FIG. 14 is a vertical sectional view showing an LED of a sixthembodiment of the present invention;

FIGS. 15A and 15B are plan views of the LED shown in FIG. 14 in whichlayers are formed up to a current blocking structure and layers areformed up to a p-side electrode, respectively;

FIGS. 16A, 16B, and 16C each show a planar configuration of a p-sideelectrode or that of a current blocking layer different from those shownin FIGS. 2, 8, 10, 13, and 15;

FIGS. 17A and 17B are a plan view and a graph, respectively, fordescribing a distance Ld from a p-side electrode to a location at whichan optical power attenuates by 90%;

FIG. 18 is an explanatory view showing the p-side electrode present at adistance of 2 Ld or less from any given point on an LED chip;

FIGS. 19A and 19B are a plan view and a graph, respectively, fordescribing a distance Ld from a current blocking layer to a location atwhich an optical power, attenuates by 90%;

FIG. 20 is an explanatory view for explaining a current blocking layerpresent at a distance of 2Ld or less from any given point on an LEDchip;

FIG. 21 is a vertical sectional view showing a conventional LED having adouble hetero-structure;

FIG. 22 is a vertical sectional view showing an LED as a background art;and

FIG. 23 shows a planar configuration of a p-side electrode shown inFIGS. 21 and 22.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(First Embodiment)

FIG. 1 a vertical sectional view showing the layer structure of the LEDof a first embodiment of the present invention. The LED shown in FIG. 1is an AlGaInP LED in which layers are lattice-matched with a GaAssubstrate 41. The structure of each layer is as follows:

-   Substrate 41:    -   made of n-type GaAs-   Buffer layer 42:    -   made of n-type GaAs-   N-type cladding layer 43:    -   made of n-type (Ga_(0.3)Al_(0.7))_(0.5)In_(0.5)P        -   impurity: Si, impurity concentration: 1×10¹⁸ cm⁻³, and        -   thickness: 1 μm-   Light-emitting layer 44:    -   made of p-type (Ga_(0.7)Al_(0.3))_(0.5)In_(0.5)P        -   thickness: 0.5 μm-   First p-type cladding layer 46:    -   made of p-type (Ga_(0.5)Al_(0.5))_(0.5)In_(0.5)P        -   impurity: Zn, impurity concentration: 1×10¹⁷ cm⁻³, and        -   thickness: 0.2 μm-   Second p-type cladding layer 47:    -   made of p-type Al_(0.5)In_(0.5)P        -   impurity: Zn, impurity concentration: 5×10¹⁷ cm³¹ ³, and        -   thickness: 1.0 μm-   First p-type current diffusion layer 48:    -   made of a p-type Ga_(0.9)In_(0.1)P        -   impurity: Zn, impurity concentration: 1×10¹⁸ cm⁻³, and        -   thickness: 1 μm-   N-type current blocking layer 49:    -   made of n-type Ga_(0.9)In_(0.1)P        -   impurity: Si, impurity concentration: 1×10¹⁸ cm⁻³, and        -   thickness: 0.5 μm-   Second p-type current diffusion layer 50:    -   made of p-type Ga_(0.9)In_(0.1)P        -   impurity: Zn, impurity concentration: 1×10¹⁸ cm⁻³, and        -   thickness: 7 μm-   Contact layer 51:    -   made of a p-type GaAs

An n-side electrode 52 is formed on the underside of the n-type GaAssubstrate 41. A p-side electrode 53 is formed on the p-type GaAs contactlayer 51.

In the LED having the structure is fabricated in the following manner.After the n-type GaAs buffer layer 42 through the n-typeGa_(0.9)In_(0.1)P current blocking layer 49 are sequentially formed onthe n-type GaAs substrate 41, the n-type Ga_(0.9)In_(0.1)P currentblocking layer 49 is partly removed to form a current blockingstructure. Then, the p-type Ga_(0.9)In_(0.1)P second current diffusionlayer 50 is laminated on the n-type Ga_(0.9)In_(0.1)P current blockinglayer 49.

In the first embodiment, MOCVD (metal organic chemical vapor deposition)method is used for the film formation. But in the present invention, thefilm growth method is not limited to the MOCVD. For example, MBE(molecular beam epitaxy) method or MCMBE (metal organic molecular beamepitaxy) method may be used.

FIG. 2 shows the LED, as viewed from above, in which the layers havebeen formed up to the current blocking structure. To collectively passelectric current through an internal portion of the LED, the currentblocking structure of the first embodiment is constructed by providingthe n-type Ga_(0.9)In_(0.1)P current blocking layer 49 so as to define acircular hole (current path) 54 therein. As in the case of the currentblocking layer 49, the p-side electrode 53 is also constructed so as todefine a circular hole (electrode window) 55 therein to take out lightemitted from the light-emitting layer 44 from the current path 54 insidethe current blocking layer 49 and the electrode window 55 inside thep-side electrode 53. In this case, the current density can be changed byvarying the size of the current path 54 to thereby improve thelight-emitting efficiency.

FIG. 3 shows dependency of the optical power of the LED on a currentdensity for different wavelengths of emitted light (namely, for colorsof emitted light). FIG. 3 indicates that the optical power is improvedby increasing the current density. In particular, because the opticalpower of a short wavelength (green) depends on the current density, itis effective to use the current blocking structure formed of the currentblocking layer 49.

As described above, in the first embodiment, the following layers aresequentially formed on the n-type GaAs substrate 41: the n-type GaAsbuffer layer 42, the n-type GaAlInP cladding layer 43, the p-typeGaAlInP light-emitting layer 44, the p-type GaAlInP first cladding layer46, the p-type GaAlInP second cladding layer 47, the p-type GaAlInPfirst current diffusion layer 48, and the n-type GaInP current blockinglayer 49. Then, to form the current blocking structure, an internalportion of the current blocking layer 49 is removed circularly to formthe current path 54. Thereafter, the p-type GaInP second currentdiffusion layer 50, the p-type GaAs contact layer 51, and the p-sideelectrode 53 are sequentially formed on the current blocking layer 49.Then, the electrode window 55 is formed by circularly removing a partdisposed immediately above the current path 54 from the contact layer 51and the p-side electrode 53.

Accordingly, electric current discharged from the p-side electrode 53formed in the periphery of the upper surface of the LED passes throughthe current path 54 inside the current blocking layer 49 and iscollectively supplied into the light-emitting layer 44. By constructingthe LED such that electric current is prevented from flowing immediatelybelow the opaque p-side electrode 53, the light take-out efficiency canbe improved. That is, according to the first embodiment, an LED having ahigh optical power can be provided.

FIG. 4 shows the optical power of the LED having the current blockingstructure formed of the current blocking layer 49 in comparison withthat of the conventional LED not having the current blocking structurefor different wavelengths of emitted light (for colors of emittedlight). FIG. 4 indicates that by providing the LED with the currentblocking structure as carried out in the first embodiment, the opticalpower of the LED having the current blocking structure is improved1.1-1.3 times as large as that of the conventional LED not having thecurrent blocking structure.

(Second Embodiment)

In the second embodiment, the thickness of the first p-type claddinglayer is larger than that of the first p-type cladding layer of thefirst embodiment to allow the LED of the second embodiment to havehigher reliability than that of the first embodiment. FIG. 5 is avertical section view showing the layered structure of the LED of thesecond embodiment. The structure of each layer is as follows:

-   Substrate 141:    -   made of n-type GaAs-   Buffer layer 142:    -   made of n-type GaAs-   N-type cladding layer 143:    -   made of n-type (Ga_(0.3)Al_(0.7))_(0.5)In_(0.5)P        -   impurity: Si, impurity concentration: 1×10¹⁸ cm⁻³, and        -   thickness: 1 μm-   Light-emitting layer 144:    -   made of p-type (Ga_(0.7)Al_(0.3))_(0.5)In_(0.5)P        -   thickness: 0.5 μm-   First p-type cladding layer 146:    -   made of a p-type (Ga_(0.5)Al_(0.5))_(0.5)In_(0.5)P        -   impurity: Zn, impurity concentration: 1×10¹⁷ cm⁻³, and        -   thickness: 0.4 μm-   Second p-type cladding layer 147:    -   made of p-type Al_(0.5)In_(0.5)P        -   impurity: Zn, impurity concentration: 5×10¹⁷ cm⁻³, and        -   thickness: 1.0 μm-   First p-type current diffusion layer 148:    -   made of p-type GaP        -   thickness: 1.0 μm-   N-type current blocking layer 149:    -   made of n-type GaP        -   thickness: 0.5 μm-   Second p-type current diffusion layer 150:    -   made of p-type Al_(0.01)Ga_(0.98)In_(0.01)P        -   thickness: 7 μm-   Contact layer 151:    -   made of p-type GaAs

An n-side electrode 152 is formed on the underside of the n-type GaAssubstrate 141. A p-side electrode 153 is formed on the p-type GaAscontact layer 151.

The film thickness of the first p-type cladding layer 146 of the LEDhaving the structure is 0.4 μm, while the film thickness of the firstp-type cladding layer 46 of the first embodiment is 0.2 μm. By makingthe first p-type cladding layer 146 thicker than the first p-typecladding layer 46, it is possible to improve the reliability of the LEDand use the LED at a higher current density. FIG. 6 shows therelationship between the film thickness of the first p-type claddinglayer 146 of the LED and the deterioration of the opticalcharacteristics of the LED when the diameter of the circular currentpath 154 is 70 μm and when electric current is passed through the LEDfor 1,000 hours at 50 mA. As obvious from FIG. 6, it is preferable thatthe thickness of the first p-type cladding layer 146 is in the range of0.2 μm to 0.5 μm. Further, an etching operation for forming the currentpath can be easily controlled by composing the first p-type currentdiffusion layer 148 and the n-type current blocking layer 149 of theGaP.

(Third Embodiment)

In the first t, the current path 54 is formed inside of the currentblocking layer 49. But the present invention is not limited to the modeof the first embodiment.

FIG. 7 is a vertical sectional view showing an AlGaInP LED having acurrent path around a current blocking layer 69. In the LED, as in thecase of the LED shown in FIG. 1, the following layers are sequentiallyformed on an n-type GaAs substrate 61: an n-type GaAs buffer layer 62,an n-type (Ga_(0.3)Al_(0.7))_(0.5)In_(0.5)P cladding layer 63, a p-type(Ga_(0.7)Al_(0.3))_(0.5)In_(0.5)P light-emitting layer 64, a p-type(Ga_(0.5)Al_(0.5))_(0.5)In_(0.5)P first cladding layer 66, a p-typeAl_(0.5)In_(0.5)P second cladding layer 67, a p-type Ga_(0.9)In_(0.1)Pfirst current diffusion layer 68, and an n-type Ga_(0.9)In_(0.1)Pcurrent blocking layer 69.

The periphery of the current blocking layer 69 is removed with itscentral part left circularly to form a current blocking structure. Then,a p-type Ga_(0.9)In_(0.1)P second current diffusion layer 70 is formedto cover the current blocking layer 69. A circular p-type GaAs contactlayer 71 and a circular p-side electrode 73 are formed on the secondcurrent diffusion layer 70 at a part thereof immediately above thecircular current blocking layer 69. An n-side electrode 72 is formed onthe underside of the n-type GaAs substrate 61.

In this case, as shown in FIG. 8 which is a view as seen from above, thep-type GaAs contact layer 71 and the p-side electrode 73 are formed in acircular shape at the center of the second current diffusion layer 70,as in the case of the conventional LED. Accordingly, the thirdembodiment has an advantage that the conventional process ofmanufacturing the p-type contact layer and the p-side electrode isapplicable to the formation of the circular p-type GaAs contact layer 71and the circular p-side electrode 73.

The present invention is not limited to the mode of the first throughthird embodiments, but can be embodied by changing the configuration ofthe current blocking layer (current blocking structure) and that of thep-side electrode.

(Fourth Embodiment)

FIG. 9 is a vertical sectional view showing an AlGaInP LED having itsemission characteristic at a large current improved by diffusingelectric current favorably and suppressing increase of a currentdensity. The structure of each layer is as follows:

-   Substrate 81:    -   made of n-type GaAs-   Buffer layer 82:    -   made of n-type GaAs-   N-type cladding layer 83:    -   made of n-type (Ga_(0.3)Al_(0.7))_(0.5)In_(0.5)P        -   impurity: Si, impurity concentration: 1×10¹⁸ cm⁻³, and        -   thickness: 1 μm-   Light-emitting layer 84:    -   made of p-type (Ga_(0.7)Al_(0.3))_(0.5)In_(0.5)P        -   thickness: 0.5 μm-   First p-type cladding layer 86:    -   made of p-type (Ga_(0.5)Al_(0.5))_(0.5)In_(0.5)P        -   impurity: Zn, impurity concentration: 1×10¹⁷ cm⁻³, and        -   thickness: 0.2 μm-   Second p-type cladding layer 87:    -   made of p-type Al_(0.5)In_(0.5)P        -   impurity: Zn, impurity concentration: 5×10¹⁷ cm⁻³, and        -   thickness: 1.0 μm-   First p-type current diffusion layer 88:    -   made of p-type Al_(0.01)Ga_(0.98)In_(0.01)P        -   impurity: Zn, impurity concentration: 1×10¹⁸ cm⁻³, and        -   thickness: 1 μm-   N-type current blocking layer 89:    -   made of n-type Al_(0.01)Ga_(0.98) In_(0.01)P    -   impurity: Si, impurity concentration: 1×10¹⁸ cm⁻³, and        -   thickness: 0.5 μm-   Second p-type current diffusion layer 90:    -   made of p-type Al_(0.01)Ga_(0.98)In_(0.01)P        -   impurity: Zn, impurity concentration: 1×10¹⁸ cm⁻³, and        -   thickness: 7 μm-   Contact layer 91:    -   made of p-type GaAs

An n-side electrode 92 is formed on the underside of the n-type GaAssubstrate 81. A p-side electrode 93 is formed on the p-type GaAs contactlayer 91.

In the fourth embodiment, a p-type cladding layer 85 is formed as atwo-layer structure consisting of the first p-type cladding layer 86 andthe second p-type cladding layer 87. Accordingly, it is possible toprevent a p-type impurity from diffusing to the light-emitting layer 84although the p-type impurity has a large impurity gradient and is liableto diffuse when electric current is passed through the LED for a longtime. Thus it is possible to prevent deterioration of the optical power.That is, in the fourth embodiment, the first p-type cladding layer 86constitutes a barrier layer.

The LED having the structure is formed in the following manner. Then-type GaAs buffer layer 82 through the n-typeAl_(0.01)Ga_(0.98)In_(0.01)P current blocking layer 89 are sequentiallyformed on the n-type GaAs substrate 81, and then, the n-typeAl_(0.01)Ga_(0.98)In_(0.01)P current blocking layer 89 is partly removedto form the current blocking structure. Then, the p-typeAl_(0.01)Ga_(0.98)IN_(0.01)P second current diffusion layer 90 is formedon the n-type Al_(0.01)Ga_(0.98)In_(0.01)P current blocking layer 89.The p-side electrode 93 is formed mediately above the current blockinglayer 89 such that the p-side electrode 93 has the almost same planarconfiguration as that of the current blocking layer 89. The lighttake-out efficiency is improved by thus blocking supply of electriccurrent to the light-emitting layer 84 disposed immediately below thep-side electrode 93 which does not transmit light.

FIGS. 10A and 10B shows the LED, as viewed from above, in a state inwhich the n-type GaAs buffer layer 82 through the current blockingstructure have been formed and in a state in which the n-type GaAsbuffer layer 82 through the p-side electrode 93 have been formed,respectively. FIG. 10A shows a planar configuration of the p-sideelectrode 93. FIG. 10B shows a planar configuration of the currentblocking layer 89. As shown in FIG. 10B, the current blocking layer 89has a planar configuration in which short strips (blocking branchportions) each having a width of 60 μm are arranged at regular intervalsof 80 μm and are connected at one end thereof to a short strip (aconnection portion) having the width of 60 μm. As shown in FIG. 10A, thep-side electrode 93 has a planar configuration in which short stripseach having a width of 30 μm are arranged at regular intervals of 110 μmand connected at one end thereof to a short strip having the width of 30μm. An LED wafer thus grown/formed is used by dividing it into chips of560 μm×560 μm (area: 0.3136 mm²).

FIGS. 11A and 11B show the relationship between the configurationcharacteristics of the current blocking layer 89 and the optical powerof the LED chip. The chip size is 560 μm×560 μm. The number of the shortstrips is two-five. FIG. 11A shows the relationship between the intervalbetween adjacent parts (namely, adjacent short strips) of the currentblocking layer 89 and the optical power of the LED chip when electriccurrent of 100 mA is passed through the LED chip. FIG. 11A indicatesthat when the interval between adjacent blocking short strips is 80 μm,the optical power of the LED chip is the highest. Examining thedistribution of the light emission intensity on the LED chip revealsthat the light emission intensity attenuates by 90% at a position spacedabout 80 μm from a short strip. Current diffusion can be considered tocorrespond to the light emission intensity. Accordingly, thelight-emitting efficiency of the LED chip becomes maxim when theinterval between two adjacent short strips of the current blocking layer89 is set to a value equivalent to a distance to the position at whichthe current intensity attenuates by 90%.

FIG. 11B shows the relationship between the percentage of the area ofthe current blocking layer 89 to the sectional area of the LED chip andthe optical power of the LED chip when electric current of 100 mA flowstherethrough. FIG. 11B indicates that it is desirable to set the area ofthe current blocking layer 89 to 30% or more of the sectional area ofthe LED chip.

As described above, the current blocking layer 89 of the fourthembodiment has the planar configuration in which short strips eachhaving a predetermined width are arranged at regular intervals and theyare connected to each other at one end thereof with another short strip.The intervals between the adjacent short strips are set to values equalto or shorter than the distance from one short strip to the position atwhich the current intensity attenuates by 90%. The p-side electrode 93is shaped to have a planar configuration in which short strips eachhaving a predetermined width shorter than that of the current blockinglayer 89 strips are arranged at predetermined intervals and connected toeach other at one end thereof with another short strip. The p-sideelectrode 93 is formed immediately above the current blocking layer 89.

When the LED is used at a large current, the current density will becometoo high. Consequently, the optical power of the LED chip will saturateand its performance and characteristics will deteriorate due to passageof electrical current. But according to the fourth embodiment, theattenuation rate of the intensity of electric current flowing the secondp-type current diffusion layer 90 is set at 90% or more. Thus, it ispossible to obtain a favorable current diffusion. That is, the LED ofthe fourth embodiment is constructed such that the current density isprevented from becoming too high in using electric current of about 100mA, which is much greater than several milliamperes to 50 mA which hasbeen used hithereto. Thus, it is possible to improve the light-emittingcharacteristic of the LED.

The distance at which the light emission strength attenuates by 90%changes according to the thickness and dope density of the second p-typecurrent diffusion layer 90. Therefore, it is necessary to optimally setthe interval between the adjacent short strips of the current blockinglayer 89, according to the changeable distance at which the lightemission intensity attenuates by 90%.

(Fifth Embodiment)

FIG. 12 is a vertical sectional view showing an AlGaInP LED having animproved light-emitting characteristic when it is used at a largecurrent owing to its construction different from that of the fourthembodiment. The structure of each layer is as follows:

-   Substrate 101:    -   made of n-type GaAs-   Buffer layer 102:    -   made of n-type GaAs-   N-type cladding layer 103:    -   made of n-type (Ga_(0.3)Al_(0.7))_(0.5)In_(0.5)P        -   impurity: Si, impurity concentration: 1×10¹⁸ cm⁻³, and        -   thickness: 1 μm-   Light-emitting layer 104:    -   made of p-type (Ga_(0.7)Al_(0.3))_(0.5)In_(0.5)P        -   thickness: 0.5 μm-   First p-type cladding layer 106:    -   made of p-type (Ga_(0.5)Al_(0.5))_(0.5)In_(0.5)P        -   impurity: Zn, impurity concentration: 1×10¹⁷ cm⁻³, and        -   thickness: 0.2 μm-   Second p-type cladding layer 107:    -   made of p-type Al_(0.5)In_(0.5)P        -   impurity: Zn, impurity concentration: 5×10¹⁷ cm⁻³, and        -   thickness: 1.0 μm-   P-type current diffusion layer 108:    -   made of p-type Al_(0.01)Ga_(0.98)In_(0.01)P        -   impurity: Zn, impurity concentration: 1×10¹⁸ cm⁻³, and        -   thickness: 7 μm-   Contact layer 109:    -   made of a p-type GaAs

An n-side electrode 110 is formed on the underside of the n-type GaAssubstrate 101. A p-side electrode 111 is formed on the p-type GaAscontact layer 109. An LED wafer thus grown/formed is used by dividing itinto chips of 560 μm×560 μm.

FIG. 13 shows the LED as viewed from above. The p-side electrode 111 hasa planar configuration in which short strips each having a width of 60μm are arranged at regular intervals of 80 μm and connected to eachother at one end thereof with another short strip having the width of 60μm. Although the LED of the fifth embodiment does not have the currentblocking layer unlike the third embodiment, the interval betweenadjacent branch electrodes (short strips) of the p-side electrode 111 isset to a value equal to or shorter than the distance at which thecurrent intensity attenuates by 90%. In this case as well, it ispossible to improve the light-emitting characteristic of the LED when itis used at a large current of about 100 mA.

(Sixth Embodiment)

FIG. 14 is a vertical sectional view showing an AlGaInP LED havingimproved light-emitting characteristic when it is used at a largecurrent owing to its construction different from that of the fourth andfifth embodiments. In the LED, as in the case of the LED of FIG. 9, thefollowing layers are sequentially provided on the n-type GaAs substrate121: an n-type GaAs buffer layer 122, an n-type(Ga_(0.3)Al_(0.7))_(0.5)In_(0.5)P cladding layer 123, a p-type(Ga_(0.7)Al_(0.3))_(0.5)In_(0.5)P light-emitting layer 124, a p-type(Ga_(0.5)Al_(0.5))_(0.5)In_(0.5)P first cladding layer 126, a p-typeAl_(0.5)In_(0.5)P second cladding layer 127, a p-typeAl_(0.01)Ga_(0.98)In_(0.01)P first current diffusion layer 128, and ann-type Al_(0.01)Ga_(0.98)In_(0.01)P current blocking layer 129.

Then, the n-type current blocking layer 129 is partly removed to form acurrent blocking structure. Then, a p-type Al_(0.01)Ga_(0.98)In_(0.01)Psecond current diffusion layer 130 is formed on the n-type currentblocking layer 129. A p-type GaAs contact layer 131 and a p-sideelectrode 133, which has a shape analogous to the current blocking layer129, are formed on the second current diffusion layer 130 at a partpositioned immediately above the current blocking layer 129. An n-sideelectrode 132 is formed on the underside of the n-type GaAs substrate121.

FIGS. 15A and 15B show the LED, as viewed from above, in a state inwhich the n-type GaAs buffer layer 122 through the current blockingstructure have been formed, and in a state in which the n-type GaAsbuffer layer 122 through the p-side electrode 133 are formed,respectively. FIG. 15A shows a planar configuration of the p-sideelectrode 133. FIG. 15B shows a planar configuration of the currentblocking layer 129. As shown in FIG. 15A, the p-side electrode 133 has aplanar configuration in which a plurality of concentric circularelectrodes are connected to each other with a connection electrode. Asshown in FIG. 15B, the current blocking layer 129 has a planarconfiguration in which a plurality of concentric circular blocking partsare connected to each other with a connection. The width of eachcircular electrode of the p-side electrode 133 is 30 μm. The intervalbetween the adjacent circular electrodes of the p-side electrode 133 is110 μm. The width of each circular blocking part of the current blockinglayer 129 is 60 μm. The interval between the adjacent circular blockingparts is 80 μm. Under these conditions, the optical power of the LED ofthe sixth embodiment is almost the same as that of the LED of the fourthembodiment. That is, when the LED is used at a large current, the effectof improving the light-emitting characteristic, which is brought aboutby the improvement of the p-side electrode and the current blockinglayer, is determined not by the shapes of the p-side electrode and thecurrent blocking layer, but by the widths of the electrode and theblocking layer and the intervals between the parts of the electrode andbetween the parts of the current blocking layer.

Accordingly, the planar shape of the p-side electrode and that of thecurrent blocking layer of the present invention are not limited to thoseshown in FIGS. 10, 13, and 15. A high light-emitting efficiency is alsoobtainable even by adopting shapes shown in FIGS. 16A-16C.

Needless to say, the size of the LED chip of the present invention isnot limited to 560 μm×560 μm. In each embodiment, the n-type claddinglayer is formed as a single layer, but may be formed as a two-layerstructure consisting of first and second n-type cladding layers. In thiscase, an n-type cladding layer closer to the light-emitting layer may beused as the barrier layer to prevent the diffusion of the n-typeimpurity to the light-emitting layer. Thereby, it is possible to obtainan LED having higher reliability.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. A light emitting diode of a double hetero-junction type in which alight-emitting layer made of a GaAlInP material is interposed between ap-type cladding layer and an n-type cladding layer, wherein: a p-sideelectrode is formed on a p-type cladding layer-side surface having anarea of 0.15 mm thus or more; any point present in a region notcontaining the p-side electrode of said p-type cladding layer-sidesurface is within a distance of (Ld×2) from some point on an edge ofsaid p-side electrode, where Ld is a distance from a position at whichan optical power is maximum, to a position at which the optical powerattenuates by 90%; and said p-side electrode comprises a plurality ofbranch electrodes and a connection electrode connecting said branchelectrodes to each other electrically.
 2. A light emitting diodeaccording to claim 1, wherein an interval between said branch electrodesis approximately Ld.
 3. A light emitting diode according to claim 2,wherein said surface on which the p-side electrode is formed has twoopposed parallel straight sides; and said branch electrodes are eachstrip-shaped, and arranged parallel with said two sides- and with eachother.
 4. A light emitting diode according to claim 3, wherein aninterval between an outermost branch electrode and the side of saidsurface opposed to this branch electrode is approximately Ld/2.
 5. Alight emitting diode according to claim 1, comprising a currentdiffusion layer made of a AIGaInP material and disposed between saidp-type cladding layer and said p-side electrode.
 6. A light emittingdiode according to claim 1, comprising a barrier layer between saidlight-emitting layer and said p-type cladding layer, said barrier layerhaving a band gap intermediate between band gaps of said light-emittinglayer and p-type cladding layer.
 7. A light emitting diode according toclaim 6, further comprising a barrier layer between said light-emittinglayer and said n-type cladding layer, said barrier layer having a bandgap intermediate between band gaps of said light-emitting layer and saidn-type cladding layer.
 8. A light emitting diode of a doublehetero-junction type in which a light-emitting layer made of a GaAlInPmaterial is interposed between a p-type cladding layer and an n-typecladding layer comprising: a current blocking layer formed on a p-typecladding layer-side surface having an area of 0.15 mm² or more; and ap-side electrode formed at a position above said current blocking layerand opposed to said current blocking layer, wherein any point present ina region not containing the current blocking layer of said p-typecladding layer-side surface is within a distance of (Ld×2) from somepoint on an edge of said current blocking layer, where Ld is a distancefrom a position at which an optical power is maximum, to a position atwhich the optical power attenuates by 90%; and said current blockinglayer comprises a plurality of blocking branch portions and a connectionportion connecting said blocking branch portions to each otherelectrically, and an interval between adjacent blocking branch portionsis approximately Ld.
 9. A light emitting diode according to claim 8,wherein the surface on which said current blocking layer is formed hastwo opposed parallel straight sides; and said blocking branch portionsare each strip-shaped and arranged parallel with said two sides and witheach other.
 10. A light emitting diode according to claim 9, wherein aninterval between an outermost blocking branch portion and the side ofsaid surface opposed to this outermost blocking branch portion isapproximately Ld/2.
 11. A light emitting diode according to claim 8,comprising a current diffusion layer made of an AlGaInP material anddisposed between said p-type cladding layer and said p-side electrode.12. A light emitting diode according to claim 8, comprising a barrierlayer between said light-emitting layer and said p-type cladding layer,said barrier layer having a band gap intermediate between band gaps ofsaid light-emitting layer and p-type cladding layer.
 13. A lightemitting diode according to claim 12, further comprising a barrier layerbetween said light-emitting layer and said n-type cladding layer, saidbarrier layer having a band gap intermediate between band gaps of saidlight-emitting layer and said n-type cladding layer.
 14. A lightemitting diode of a double hetero-junction type in which alight-emitting layer made of a GaAlInP material is interposed between ap-type cladding layer and an n-type cladding layer, wherein: a p-sideelectrode is formed on a p-type cladding layer-side surface, said p-sideelectrode consisting of a plurality of mutually connected constituentparts; and any point present in a region not containing the p-sideelectrode of said p-type cladding layer-side surface is within adistance of (Ld×2) from some point on an edge of said p-side electrode,where Ld is a distance from a position at which an optical power ismaximum, to a position at which the optical power attenuates by 90%. 15.A light emitting diode according to claim 14, wherein said p-typecladding layer-side surface is a surface of a current diffusion layer.16. A light emitting diode according to claim 15, wherein the currentblocking layer having a shape corresponding to that of said p-sideelectrode is formed inside said current diffusion layer in a positionopposed to said p-side electrode.